Cascaded power plant using low and medium temperature
source fluid

ABSTRACT

The present invention provides a method for operating a plurality of independent, closed cycle power plant modules each having a vaporizer comprising the steps of serially supplying a medium or low temperature source fluid to each corresponding vaporizer of one or more first plant modules, respectively, to a secondary preheater of a first module, and to a vaporizer of a terminal module, whereby to produce heat depleted source fluid; providing a primary preheater for each vaporizer; and supplying said heat depleted source fluid to all of said primary preheaters in parallel.

FIELD

The present invention relates to the field of power plants. Moreparticularly, the invention relates to a cascaded closed Rankine cyclepower plant.

BACKGROUND

Low and medium temperature source fluids, hereinafter termed sourcefluids of the type described, are those fluids with a temperature lessthan about 177° C. (350° F.), such as geothermal fluids obtained frommany production wells, and industrial liquids produced by variousindustrial processes. The East Mesa Development Project located in theImperial Valley of Southern California near Holtville has been producingabout 4 million pounds per hour of geothermal fluid at about 162° C.(324° F.). Such geothermal fluid is an example of source fluid of thetype described.

Electricity is generally produced from source fluids of the typedescribed using a closed Rankine cycle heat engine whose operating fluidis an organic fluid (e.g., Freon), such system being termed a powerplant of the type described. A source fluid of the type described isapplied to a vaporizer of a power plant of the type described containingliquid organic fluid whereby the latter is converted into a vapor. Thevapor is expanded in a turbogenerator that converts some of the heat inthe vapor to work and produces heat depleted organic vapor that iscondensed in a condenser. The condensed organic fluid is returned to thevaporizer, and the cycle is repeated.

The condenser rejects the remaining heat in the heat depleted vapor intoambient air, if an air-cooled condenser is involved, or into coolingwater, if a water-cooled condenser is used. Typically, the vaporizer isoperated at a pressure that produces saturated or only slightlysuperheated vapor because the pressures involved are relatively low andthe design of the heat exchanger that constitutes the vaporizer, thepiping for conveying the vapor, and the turbine, are simplified. Inorder to maximize power output of a power plant of the type described,the temperature drop of the source fluid across the entire heatexchanger system of the power plant, and the evaporization temperaturein the vaporizer must be optimized.

Prior art cascaded power plants utilizes a plurality of closed Rankinecycle power plant modules each having an associated heat exchanger, thesource fluid being serially applied to the heat exchangers of eachmodule. Whatever system is used, maximizing the net power produced bythe system is of paramount importance. One technique for increasing thepower is to extract more heat from the source fluid by increasing itstemperature drop. With either a single stage or cascaded system,however, increasing the amount of heat extracted from the source fluidby increasing the temperature drop of the source fluid across the heatexchanger system has the effect of decreasing efficiency of the powerplant because the mean temperature of the source fluid is reduced. Thisresults in a reduction of the evaporization temperature of the operatingfluid in the heat exchanger, thus reducing the Carnot efficiency of thepower plant.

In another method for increasing the efficiency level of a power plant,a prior art power plant is operated by serially applying the sourcefluid to the vaporizers of the modules for producing heat depletedsource fluid. A preheater is provided for each vaporizer, and the heatdepleted source fluid is applied to all of the preheaters in parallel.

In an effort to increase the efficiency of a power plant of the typedescribed, and to extract more power from the source fluid, it has beenproposed to operate at super critical temperatures and pressures. Insuch case, the temperature of the vaporized organic fluid produced bythe heat exchanger system is higher than in the above-described typicalRankine cycle power plant. While this approach is effective to increasethe efficiency of the power plant and to increase its work output, thegains are offset by the higher cycle pump power consumption, as well asincreased cost and complexity of the power plant whose pressure vesselsmust be designed to operate at pressures in the range of 500-600 psia.

Consequently, the present invention provides a cascaded closed Rankinecycle power plant using low and medium temperature source fluid whichadvantageously can produce an increased power level relative to thatproduced by prior art power plants.

Other advantages of the invention will become apparent as thedescription proceeds.

SUMMARY

The present invention provides a method for operating a plurality ofindependent, closed cycle power plant modules each having a vaporizercomprising the steps of:

(a) serially supplying a medium or low temperature source fluid to eachcorresponding vaporizer of one or more first plant modules,respectively, to a secondary preheater of a first module, and to avaporizer of a terminal module, whereby to produce heat depleted sourcefluid;

(b) providing a primary preheater for each vaporizer; and

(c) supplying said heat depleted source fluid to all of said primarypreheaters in parallel.

In one aspect, the source fluid is geothermal fluid.

In one aspect, the source fluid is fluid generated from an industrialprocess.

In one aspect, each of the power plant modules is operated at differenttemperatures.

In one aspect, each of the power plant modules is operated at differentpressures.

In one aspect, the motive fluid for the power plant modules is organicfluid.

In one aspect, the same type of motive fluid is used in each module.

In one aspect, each module is based on a Rankine cycle.

The present invention is also directed to a power plant of the typehaving a plurality of independent, closed cycle power plant modules eachof which comprising a vaporizer to which a medium or low temperaturesource fluid is serially applied for producing heat depleted fluid, anda primary preheater for each of said vaporizers, each of said primarypreheaters adapted to preheat motive fluid condensate by means of saidheat depleted fluid which is supplied to all of said preheaters inparallel, the improvement comprising a secondary preheater to which saidsource fluid is serially applied from a first vaporizer and from whichsaid source fluid is supplied to a terminal vaporizer, said secondarypreheater adapted to preheat motive fluid condensate exiting from afirst primary preheater before being introduced to a corresponding firstvaporizer.

Each module advantageously comprises a vaporizer responsive to thesource fluid for converting the motive fluid condensate to vapor; aturbogenerator responsive to motive fluid vapor produced by saidvaporizer for generating power and producing expanded motive fluidvapor; and a condenser for condensing said expanded motive fluid andproducing liquid motive fluid condensate that is supplied to the primarypreheater associated with said vaporizer.

The condenser may be water cooled or air cooled.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a block diagram of a cascaded power plant, according to oneembodiment of the present invention;

FIG. 1A is a block diagram of a cascaded power plant, according toanother embodiment of the present invention;

FIG. 2 shows an example of temperature-heat diagrams for the power plantmodules and

FIG. 3 is a block diagram of a cascaded power plant, according to theembodiment of the present invention described with reference to FIG. 3.Note that similar reference numerals refer to similar components.

DETAILED DESCRIPTION

The present invention is an improved cascaded power plant using low andmedium temperature source fluid (hereinafter the “source fluid”). Oneprior art power plant is operated by serially applying the source fluidto the vaporizers of the modules for producing heat depleted sourcefluid. A preheater is provided for each vaporizer, and the heat depletedsource fluid is applied to all of the preheaters in parallel. Such powerplant systems are described in U.S. Pat. No. 4,578,953, the disclosureof which is incorporated by reference. Furthermore, U.S. Pat. No.4,700,543 discloses a similar cascaded power plant having a plurality ofmodules each of which being arranged in a plurality of levels. Thedisclosure of U.S. Pat. No. 4,700,543 is also incorporated by reference.In the present invention, an additional preheater is applied to one ofthe vaporizers. The temperature of the corresponding vaporized organicmotive fluid is therefore increased, enabling more vapors to beextracted and to increase the power output of the power plant by theorder of about 1-2%.

In addition, U.S. Pat. No. 5,531,073 discloses a similar cascaded powerplant having a plurality of modules each of which being arranged in aplurality of levels. The disclosure of U.S. Pat. No. 5,531,073 is alsoincorporated by reference.

FIG. 1 illustrates a cascaded power plant generally designated as 10,according to one embodiment of the present invention. Power plant 10comprises a plurality of independent closed, Rankine cycle organic fluidpower plant modules e.g. module 5A, module 5B and module 5C. Three suchpower plant modules are shown; but the invention is applicable to two ormore independent power plant modules. Each of these modules is similarand as a consequence, only module 5C is described in detail.

Module 5C has a piping system 3C indicated by a thick line, throughwhich the organic fluid circulates. Heated organic liquid is deliveredto vaporizer 13C and is vaporized by means of heat from the source fluidintroduced from inlet I and flowing through source fluid piping system11. The organic liquid contained within vaporizer 13C is vaporizedproducing essentially saturated or slightly superheated vapor which isapplied to turbine 16C of turbogenerator 15C. The vapor expands inturbine 16C, and work is produced so that electric generator 17C drivenby turbine 16C produces electric power. The vapor exhausted from turbine16C is applied to condenser 18C wherein the vapor is condensed intoliquid by the application to the condenser of cooling water that flowsthrough line 9C. Alternatively, an air cooled condenser can be used.

By means of a pump (not shown), condensate produced by condenser 18C issupplied via line 3C into preheater 19C that may be a physical part of,or separate from vaporizer 13C. Heat depleted source fluid, obtainedfrom the outlet from vaporizer 13C, is applied to preheater 19C, to heatthe organic fluid condensate. If the source fluid is geothermal, thecooled source fluid that exits preheater 19C may be supplied to arejection well; or, if the source fluid is an industrial chemical, thecooled fluid may be transferred back to the process. The organic fluidthat is heated in pre-heater 19C by the heat depleted source fluid isdelivered to vaporizer 13C.

After being injected into piping system 11 at inlet I, the source fluidis first delivered to vaporizer 13A of module 5A. The source fluid thatexits vaporizer 13A is delivered to vaporizer 13B of module 5B, and thesource fluid that exits from vaporizer 13B is applied to intermediatepreheater 19A1 of module 5A. Advantageously, preheater 19A1 can beportion of vaporizer 13A where it can operate as a preheater zone.Thereafter, the source fluid that exits intermediate preheater 19A1 isdelivered to vaporizer 13C of module 5C. The source fluid that exitsfrom vaporizer 13C is termed heat depleted source fluid because of theheat extracted from each of vaporizers 13A, 13B and 13C as well aspreheater 19A1. This heat depleted fluid is applied to each of thepreheaters 19A2, 19B and 19C, in parallel. That is to say, the presentinvention provides for serially applying a source fluid from inlet I tovaporizer 13A, vaporizer 13B, intermediate preheater 19A1, and vaporizer13C and for applying heat depleted source fluid to each preheater 19A2,19B, and 19C in parallel. The source fluid that exits from each of thepreheaters 19A2, 19B, and 19C can be conveyed, as shown, to a rejectionwell if the source fluid is geothermal. With respect to module 5A, themotive fluid condensate produced by condenser 18A is delivered to firststage preheater 19A2 via line 3A, additionally heated by intermediatepreheater 19A1 and then vaporized by vaporizer 13A and the motive fluidvapor produced is supplied to vapor turbine 16A for producing powerusing electric generator 17A run by vapor turbine 16A. Alternatively, arecuperator can be used for utilizing heat present in the organic vaporexiting vapor turbine 16A to heat motive fluid condensate produced bycondenser 18A before it is delivered to first stage preheater 19A2. Inaddition, alternatively, an electric generator can be used for producingelectric power from vapor turbines 16A and 16B. Furthermore, arecuperator can also be used in power plant module 5B so that organicvapor exiting vapor turbine 16B heats motive fluid condensate producedby condenser 18B before it is delivered to preheater 19B. In such acase, less heat can be extracted from the heat depleted heat sourcefluid. This can be advantageous particularly which geothermal fluid suchas liquid or brine is used as the heat source fluid since, under such asituation, a further power plant module can be used to utilize heatstill present therein.

FIGS. 2A, 2B and 2C illustrate an example of a typical temperature-heatdiagram for the three power plant modules 5A-C shown in FIG. 1. Fromthese Figures it can be seen that due to the additional pre-heatingstage or pre-heater used in power plant module 5A, a higher boiling orvaporizing temperature can be achieved. As a consequence, a higheroverall power plant efficiency level can be achieved in power plantmodule 5A.

In situations where only two power plant modules are used, it can beadvantageous to use the secondary preheater of the first power plantmodule such that the medium or low temperature source fluid exiting thevaporizer in the first power plant module be supplied to the secondarypreheater of the first power plant module prior to supplying it to thevaporizer of the second. power plant module (see FIG. 1A). In such amanner, the heat transfer from the medium or low temperature sourcefluid can be optimized. Moreover, steam, e.g. geothermal steam, can beused in the secondary preheater of the first power plant module tocompetently increase the amount of heat transferred to the motive fluidin the secondary preheater of the first power plant module. Also, inthis embodiment, heat depleted medium or low temperature source fluidexiting the vaporizer in the second power plant module can beadvantageous supplied in parallel to the primary preheaters in the firstpower plant and second module. Furthermore, also in this embodiment,either water cooled or air cooled condensers can be used. In addition,in such a case, two single pass condensers connected in series can beused in each of the first and second power plant modules (see FIG. 3) tofacilitate and optimize operation of the power plant modules atrelatively low temperatures of the medium or low temperature sourcefluid (e.g. about 260° F.) particularly when using an organic motivefluid such as butane, e.g. n-butane, iso-butane etc. In this example,air cooled condensers are referred to. In such two single passcondensers connected in series, the use of drainage of the motive fluidcondensate between the first pass and the second pass shown in FIG. 3 isadvantageous in providing improved heat exchanger performance for suchcondensers. Note that while it is shown on FIG. 3 that each power plantmodule has a separate electric generator, advantageously, both turbinesT1′ and T2′ respectively can run a joint electric generator which can beinterposed between the turbines. Furthermore, while 260° F. is given asan example of relatively low temperatures of the medium or lowtemperature source fluid, in accordance with this embodiment of thepresent invention, temperatures of between e.g. about 235° F. to 280° F.can also be used. When the temperatures is about 235° F., it could beadvantageous to use a single power plant module using an organic motivefluid such as butane, e.g. n-butane, iso-butane, having a condenser(advantageously air-cooled) comprising two single-pass condenserportions connected in series in the power plant module which couldinclude as well a preheater in addition to the vaporizer.

In addition, while butane, e.g. n-butane, iso-butane, etc. are mentionedas examples of motive fluids especially for use with relatively lowtemperatures of the medium or low temperature source fluid,advantageously organic motive fluids having a boiling point betweenabout −35° C. to 20° C. can be used as motive fluids for such relativelylow temperatures medium or low temperature heat source fluids.

While some embodiments of the invention have been described by way ofillustration, it will be apparent that the invention can be carried outwith many modifications, variations and adaptations, and with the use ofnumerous equivalents or alternative solutions that are within the scopeof persons skilled in the art, without departing from the spirit of theinvention or exceeding the scope of the claims.

1. A method for operating a plurality of independent, closed cycle powerplant modules each having a vaporizer comprising the steps of: (a)serially supplying a medium or low temperature source fluid to eachcorresponding vaporizer of one or more first plant modules,respectively, to a secondary preheater of a first module, and to avaporizer of a terminal module, whereby to produce heat depleted sourcefluid; (b) providing a primary preheater for each vaporizer; and (c)supplying said heat depleted source fluid to all of said primarypreheaters in parallel.
 2. A method according to claim 1, wherein thesource fluid is a geothermal fluid.
 3. A method according to claim 1,wherein each of the power plant modules is operated at differenttemperatures.
 4. A method according to claim 3, wherein each of thepower plant modules is operated at different pressures.
 5. A methodaccording to claim 1, wherein the motive fluid for the power plantmodules is an organic fluid.
 6. A method according to claim 5, whereinthe same motive fluid is used in each module.
 7. A method according toclaim 1, wherein each module is based on a Rankine cycle.
 8. In a powerplant of the type having a plurality of independent, closed cycle powerplant modules each of which comprising a vaporizer to which a medium orlow temperature source fluid is serially supplied for producing heatdepleted fluid, and a primary preheater for each of said vaporizers,each of said primary preheaters adapted to preheat condensed motivefluid by means of said heat depleted fluid which is supplied to all ofsaid preheaters in parallel, the improvement comprising a secondarypreheater to which said source fluid is serially applied from a firstvaporizer and from which said source fluid is supplied to a terminalvaporizer, said secondary preheater adapted to preheat motive fluidcondensate exiting from a first primary preheater before beingintroduced to a corresponding first vaporizer.
 9. The power plant ofclaim 8, wherein the motive fluid is an organic fluid.
 10. The powerplant of claim 8, wherein the source fluid is a geothermal fluid. 11.The power plant of claim 8, wherein each power plant module is a closedRankine cycle power plant module.
 12. The power plant of claim 11,wherein each module comprises; (a) a vaporizer responsive to the sourcefluid for producing motive fluid vapor; (b) a turbogenerator responsiveto motive fluid vapor produced by said vaporizer for generating powerand producing expanded motive fluid; and (c) a condenser for condensingsaid expanded motive fluid and producing liquid motive fluid condensatethat is supplied to the primary preheater associated with saidvaporizer.
 13. The power plant according of claim 12, wherein thecondenser is air cooled.
 14. The power plant according to claim 13wherein two power plant modules are included.
 15. The power plantaccording to claim 14 wherein said condenser of each said power plantmodules comprises two single-pass condensers connected in series.